This invention generally relates to systems for tracking propagation delays associated with various communications through a central office switch, or other comparable communication node, in a public telephone network. In particular, the invention provides methods and apparatus for accurately determining a propagation delay associated with a particular communication transaction in a network having common channel signaling, such as Signaling System 7 (SS7), by assessing propagation delays associated with the provision of a particular bearer service, transmission path delays, impacts depending on an information type transmitted and transmission devices involved, or any combination of these.
A telecommunications environment having a common signaling protocol is one in which central office exchanges share a common interfacing language. Hence, no translation or interpretation of messages between exchanges is required. SS7 is a common signaling protocol that has been globally adopted. An SS7 network contains dedicated signaling channels that are separate from voice or data paths or links and that are used for control messaging. This is illustrated in FIG. 1, which shows a separate speech connection and signaling network for carrying speech data and control messages, respectively, between exchanges A and B. Although the signaling messages are separated from the speech, each signaling message is associated with a specific speech connection. All signaling messages for setting up and releasing a call travel through a dedicated signaling channel, that may be shared by signaling messages for several calls.
The signaling channel can be physically separated from the speech path, i.e., take a different route through the communication network. As a result, a propagation delay procedure such as that described here is needed. Since signaling messages can be routed a different way through the communication network, propagation delay measurements on a signaling path are typically not useful for a speech path.
A call in an SS7 supported network is basically completed as follows. A calling party dials the phone number of a called party. The dialed digits are received by the originating exchange and an Initial Address Message (IAM) is generated that contains the dialed digits and other information. The IAM is routed through the network to the terminating exchange based on dialed digits and transmission requirements. When the IAM is routed through the network, it may pass through one or more intermediate exchanges. When the IAM is received at the terminating exchange, the called party's line is checked. The called party's line status (busy/idle) is returned back to the originating exchange. If the line is busy, the call is released; if the line is idle, the subscriber is alerted in an appropriate way. If there is an answer, the terminating exchange sends an answer or connect message back toward the originating exchange, at which point call set-up is completed. The call is then in a so-called active phase, in which the originating party and the terminating party have successfully entered into a connection with one another.
A communication transaction in a telecommunication environment is any part of a call set up or call completion. A communication transaction may involve any communication means such as a central office exchange, toll or gateway exchange, land based communication node, cellular base station, microwave or satellite link, or any other connection between the aforementioned, or like, communication devices.
Propagation delay is that amount of time required by a signal to travel from a source to a receiver. Propagation delay information is accumulated during call set-up (communication set-up) in a forward direction (i.e., from the originating exchange toward the terminating exchange). The accumulated propagation delay information is sent back toward the originating exchange before the call enters the active phase.
The assessment of the propagation delay information is based on increasing a propagation delay counter (PDC) contained in the IAM as the call is set up. The accumulated result is sent in a backward direction (back to the originating exchange) within a call history information parameter in the answer or connect message.
In accordance with the International Telecommunication Union standard ITU-T Q.764 for SS7 standards regarding ISDN User Part (ISUP) signaling procedures, propagation delay information associated with a communication transaction is to be maintained. An initiating exchange includes the propagation delay counter in the IAM. Initially, the counter is set to zero milliseconds (ms), or if a propagation delay value is known (having been predetermined for each of the various circuits available to the initiating exchange), the counter is set to that value. Depending on the route (circuit) selected to the terminating or succeeding exchange, the originating or preceding exchange increases the PDC by the value representing the appropriate delay value (for the selected circuit) prior to sending the IAM. The value of the delay for each circuit is typically maintained in an exchange. If the chosen circuit includes a satellite link, a satellite indicator is set accordingly.
Upon receipt of the call history information parameter in the answer or connect message, the initiating exchange stores the received data value until the call is released. If an intermediate exchange (between the originating and terminating exchanges) is involved in the call, such as a national transit exchange, outgoing gateway exchange, toll exchange, or international outgoing exchange, appropriate adjustment of the PDC is required in the intermediate office for the particular circuits selected.
Upon receipt of the IAM, the intermediate exchange analyzes IAM routing information, including PDC information, in order to select an appropriate circuit for connection to a succeeding exchange. After such a circuit is selected, the propagation counter is increased by a value corresponding to the delay value associated with the selected circuit. This value is made available in the intermediate exchange. The IAM is sent to the succeeding exchange containing the new value of the PDC. When the call enters the active phase, the intermediate exchange relays the received answer or connect message back toward the originating exchange which message includes the call history information parameter, and consequently the accumulated PDC value.
A terminating exchange is the exchange (e.g., the destination central office) in which the PDC assessment is completed. Upon receipt of the IAM in the terminating exchange, the exchange increases the PDC by the delay value of a terminating access, if available, and stores the PDC until the call is released. The terminating exchange, prior to sending the answer or connect message back to a preceding exchange, updates the call history information parameter according to the stored value of the propagation delay counter.
In addition to communications between exchanges having common protocols, there are two basic interworking scenarios where offices having different signaling protocols interact. The first scenario is where an office that provides a signaling system that supports the PDC update procedure (supported) initiates a call set up with an office having a signaling system that does not support the PDC update procedure (unsupported). The second scenario is where an unsupported office initiates a call set up with a supported office.
In the case of an interworking situation where a supported office initiates a call to an unsupported office, the supported exchange stores the propagation delay value accumulated up to the present point, until the call is released. If a delay value referring to the unsupported part of the connection is available, the delay is added to the delay value stored in the supported exchange. Otherwise, the supported exchange acts as a terminating exchange. Upon receipt of an answer or connect message back from the unsupported office, the supported exchange includes the call history information parameter set to the stored delay value. The answer or connect message, including the call history information parameter, is sent to the preceding exchange by the supported exchange.
In the case of an interworking situation where an unsupported office initiates a call to a supported office, the unsupported exchange typically does not have signaling containing a propagation delay counter. In such a case the supported exchange inserts the propagation delay counter in the IAM resetting it to zero ms. If a delay value referring to the unsupported part of the connection is available, the propagation delay counter shall be set to this delay value. Otherwise, the supported exchange acts as an initiating exchange. The calculated delay value in the supported exchange reflects the delay incurred on the preceding circuit(s). This includes delay values corresponding to satellite delays, if applicable. Prior to sending an IAM to a succeeding exchange, the propagation delay counter in the supported exchange is increased according to the delay value of the circuit selected.
In the situation where the incoming exchange is supported, but where the IAM does not contain the propagation delay counter parameter, a propagation delay value is created just as it would be when there is interworking from an unsupported exchange to a supported exchange. If a supported exchange receives an answer or connect message from which the call history information parameter is missing, no special action is required as the answer or connect message is sent backward without this parameter. If a supported exchange receives a confusion message referring to the PDC, or to the call history information parameter, both the confusion message and any stored propagation delay value are discarded.
A confusion message (CFN) is an SS7 message that indicates that unrecognized information has been received. Because an SS7 exchange supports interworking with different versions of SS7 exchanges, the SS7 exchange may receive information that is not within the exchange's communication protocol. The CFN message is used to inform the sending exchange that it has sent information that the receiving exchange could not understand. The CFN message indicates what action, if any, was taken (e.g., whether the received information was passed on or discarded), as well as information about what type of information was not understood.
In the above-described ITU-T propagation delay procedures, the delay value is set solely according to the delay associated with a selected circuit. Such a procedure fails to take into consideration the fact that different delays can exist on the same circuit depending on the particular bearer service being provided. Bearer services include voice, data, and video communications. The above identified standard also neglects a number of additional factors that may impact the propagation delay value. These include delays associated with the utilization of supplementary network equipment in combination with bearer services. Hence, the ITU-T procedures mandate a propagation delay tracking standard in which the propagation delay information may be inaccurate, or at best, a crude estimate of an actual value.
Accurate propagation delay information is needed for a number of reasons, including provision of a determination factor for utilizing echo suppression systems or other signal processing to maintain communication signal quality. Accurate propagation delay information also can facilitate enhanced routing determination, more effective selection of circuits, and more efficient use of network resources. If propagation delay becomes too great, there is degradation in communication quality. Hence it is desirable to maintain propagation delay at a manageable level.
For instance when a call having a relatively high propagation delay value is received in an exchange, it is desirable that a circuit with a relatively low delay value be selected to avoid unduly increasing the accumulated propagation delay. Conversely, when a call whose propagation delay value is relatively low is received, it may be acceptable to select a circuit with a relatively high delay value, thus saving low delay circuits for use in high delay calls. Hence, in order to make the most efficient use of exchange resources without degrading communication quality and to avoid reliance on undue echo suppression or signal processing, the most accurate propagation delay information should be maintained.
In accordance with the invention, methods and apparatus are provided for maintaining more accurate propagation delay information compared to that prescribed by the ITU-T standards discussed above.